An approach is suggested, invoking vitally the notion of constituent massive quarks (valons) which can survive and propagate rather than hadrons (except of pions) within the hot and dense matter formed below the chiral transition temperature in course of the heavy ion collisions at high energies. This approach is shown to be quite good for description of the experimentally observed excess in dilepton yield at masses 250 MeV ≤ M ee ≤ 700 MeV over the prompt resonance decay mechanism (CERES cocktail) predictions. In certain aspects, it looks to be even more successful, than the conventional approaches: it seems to match the data somewhat better at dilepton masses before the two-pion threshold and before the ρ-meson peak as well as at higher dilepton masses (beyond the φ-meson one). The approach implies no specific assumptions on the equation of state (EOS) or peculiarities of phase transitions in the expanding nuclear matter.

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Contents 1. Introduction 2. Retrospect, heuristic considerations, and QCD 3. The phase plane: the current outlook 4. Theoretical models 4.1 Lattice calculations; 4.2 Bag model; 4.3 Interim results: what do the theoretical models teach us? 5. Man-made subhadronic matter? 5.1 A general view of the process; 5.2 The direct phase transition scenario, QGP -> H: pro and contra; 5.3 The scenario with two phase transitions, QGP -> QπK -> H: advantages and problems; 5.4 Dilepton (e + e~-pairs) production Concluding remarks ReferencesAbstract. Major aspects of the subhadronic state of nuclear matter populated with deconfined color particles are reviewed. At high and even at rather low nuclear collision energies, this is expected to be a short-term quark-gluon plasma (QGP), but, seemingly, not only this. Emphasis is put on the self-consistency requirement that must be imposed on any phenomenological description of the evolution of a hot and dense nuclear medium as it expands (cools down) to the point where the final scattering of secondary particles starts. The view is argued and analyzed that massive constituent quarks should then play a major role at a certain cooling stage. A hypothesis is discussed regarding the existence of an intermediate stage (a valon plasma), allowing a consistent explanation of data on the mid-rapidity yields of various kinds of hadrons and direct dileptons (e + e -pairs) in high-energy heavy-ion collisions.

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